Integrated optoelectronics devices

ABSTRACT

The invention is an optoelectronic device and method of fabrication where at least two optical devices are formed on a single semiconductor substrate, with each optical device including an active region such as a multi-quantum well region. The active devices are spatially separated and optically coupled by a passive waveguide formed over the substrate which provides butt joints with the active regions. The butt joints can be optimized independently from the active regions thus improving yield.

FIELD OF THE INVENTION

[0001] This invention relates to the field of optoelectronics, and inparticular to a structure and method which permits integration of atleast two optoelectronic devices on a single substrate.

BACKGROUND OF THE INVENTION

[0002] Optical systems are currently the subject of a great deal ofattention in telecommunications primarily due to their enormousinformation-handling capacity. A typical system includes, at thetransmitter end, a source of light, such as a laser, a modulator forimpressing information onto the light signal, and one or more opticalamplifiers for amplifying the optical signal. The signal is usuallytransmitted by means of an optical fiber. At the receiver end, typicallya photodetector such as a PIN diode or avalanche photodiode (APD) may beemployed to convert the optical signal to an electrical signal. Ofcourse, several other components, such as optical switches, circulators,and isolators may be employed.

[0003] For purposes of economy and size it is desirable to integrate asmany devices as possible on a single substrate. For example, it is knownto integrate a laser and modulator into a single device generally knownas an Electroabsorption Modulated Laser (EML). It is also known tointegrate a Distributed Bragg Reflection (DBR) laser, a modulator, aSemiconductor Optical Amplifier (SOA) and a monitor onto a singlesubstrate. Normally, when integrating active device components (i.e.,devices which provide optical gain), the devices are formed by SelectiveArea Growth (SAG) with modifications to the composition, thickness, ornumber of Quantum Well layers in the active regions of the variousdevices. In the case of two active devices, it is difficult to optimizethe device characteristics and the butt joint which couples the twodevices together. In the case of three or more devices, the problem ofoptimization becomes especially difficult.

[0004] It has been proposed to couple active devices in an integratedstructure using a passive waveguide. (See, e.g., U.S. Pat. No. 5,134,671issued to Koren et al, and U.S. Pat. No. 5,029,297 issued to Halemane,et al.) However, such devices provide coupling between waveguide anddevice in a vertical direction which can also be difficult to make sincethe waveguide needs to be formed in the same growth process as theactive devices. It is generally more advantageous to provide a buttcoupling between devices (i.e., the light coupling is done in ahorizontal direction) so that the coupling is optimized independentlyfrom the active devices.

[0005] It is desirable, therefore, to provide an integratedoptoelectronic device with at least two active components where devicecharacteristics and butt joint coupling may be optimized.

SUMMARY OF THE INVENTION

[0006] The invention in accordance with one aspect is an optoelectronicdevice comprising at least two optical devices formed on a singlesemiconductor substrate, each optical device including an active region.The active devices are spatially separated and optically butt coupled bya passive waveguide formed on the substrate.

[0007] In accordance with another aspect, the invention is a method offorming an optoelectronic device comprising the steps of forming aplurality of epitaxial semiconductor layers on essentially the entiresurface of a semiconductor substrate, the layers including at least onelayer of an active material. The layers are then selectively etched toform spatially separate structures including the active material. Anadditional plurality of layers are then formed in the spaces between thestructures, the additional layers including at least one passivewaveguide layer so as to provide optical butt coupling between theactive material of the separate structures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] These and other features of the invention are delineated indetail in the following description. In the drawing:

[0009]FIG. 1 is a cross sectional view of one example of a prior artoptoelectronic device;

[0010]FIG. 2 is a cross sectional view of an optoelectronic device inaccordance with one embodiment of the invention; and

[0011]FIGS. 3-8 are cross sectional views of the device of FIG. 2 duringvarious stages of manufacture in accordance with an embodiment of themethod aspects of the invention.

[0012] It will be appreciated that, for purposes of illustration, thesefigures are not necessarily drawn to scale.

DETAILED DESCRIPTION

[0013] One example of a prior art device is illustrated in FIG. 1.Merely by way of illustration, the device includes three active devices,a laser, 10, a modulator, 11, and a semiconductor optical amplifier(SOA), 12, formed on a semiconductor substrate, 13. Many othercomponents can also be included, such as photodetectors. Each deviceincludes a cladding or combination of cladding and Separate Confinementlayers, 14, formed on the surface of the substrate, 13, an active layer,15, deposited on the cladding layer, and another cladding/SCL layer, 16,deposited on the active layer. Other layers typically used, such asbuffer layers and contact layers, are not shown for the sake of clarity.It will also be appreciated that the active layer, 15, can comprise asingle Multi Quantum Well (MQW) layer or a plurality of MQW layersseparated by barrier layers (not shown) as known in the art. Electrodes,16-18, are selectively formed over the device structures, and electrode19 is typically formed over the opposite surface of the substrate.

[0014] Typically, such devices are formed by depositing the variouslayers by Selective Area Growth (SAG) techniques. Varying the shape orsize of the mask along the substrate can produce variations incomposition and/or thickness of the active layer, 15, in the deviceregions, 10, 11, and 12, with a single deposition resulting in thedesired device performances. The deposition also results in butt joints,illustrated by lines 20-23, which provide horizontal coupling betweenthe devices. Thus, the device of FIG. 1 includes an active layer, 15,which constitutes the active regions of the individual devices, 10-12,as well as the interconnection between the devices (although thecomposition and/or thickness of the layer, 15, will be varied across thedevice structure). A problem with such a technique is the difficulty infinding a growth condition which will optimize all the devices, 10-12,as well as the butt joints, 20-23. This problem is especially acute whenthe active regions of the devices comprise one or more MQW layers, andthe integrated device is intended for use at 10 Gbit for distances of atleast 50 km or for use at 40 Gbit.

[0015]FIG. 2 illustrates a device structure in accordance with anembodiment of the invention. Again, the device, 30, includes a laser,31, a modulator, 32, and an SOA, 33, integrated onto a single substrate,33. As before, each component, 31-33, comprises a cladding/SCL layer,35, an active layer, 36, and another cladding/SCL layer 37. However,rather than provide interconnection of the components by means of anactive waveguide, the interconnections are made by a structure, 41 and42, which includes a passive waveguide layer, 43. In this particularexample, the passive waveguide layer, 43, is sandwiched between SCLlayers, 44 and 45. In a preferred embodiment, the layers 43-45, can beformed sequentially in all the areas between components at the sametime, i.e., the same layers will comprise the interconnections betweenall components. The device, 30, also preferentially includes a layer,46, formed over essentially the entire substrate surface. The layercomprises a composition, such as InGaAsP, which can perform the functionof a stop etch in the processing to be described.

[0016] Another distinction in the device of FIG. 2 is the fact that theactive and cladding/SCL layers of any component, 31-33, are notnecessarily formed at the same time as the corresponding layers of anyother component, and, therefore, can be independently optimized.

[0017] It should be appreciated that one of the advantages of thestructure of FIG. 2 is that the butt joints, 50-54, formed by thepassive waveguide layer, 43, and SCL layers, 44 and 45, can be optimizedindependently from the optimization of the active regions of thecomponents, 31-33. Further, since the passive waveguide layer istypically undoped, improved electrical isolation and reduced opticalloss can be achieved.

[0018] It should be understood that in the context of the presentapplication, an active waveguide layer is considered to be anysemiconductor layer which will generate light or absorb light inresponse to an applied bias. A passive waveguide layer is considered tobe any semiconductor layer which will channel the light withoutgenerating any light or absorbing any significant amount of the light(less than 0.1 dB loss).

[0019]FIGS. 3-8 illustrate a sequence of steps which can be performed inaccordance with an embodiment of the method aspects of the invention inorder to produce the device of FIG. 2. In this particular example, allsemiconductor layers are formed by Metal Organic Chemical VaporDeposition (MOCVD), but other known techniques can be employed.

[0020] As illustrated in FIG. 3, the substrate in this example comprisesInP. Formed over essentially an entire major surface of the substrate isa stop etch layer, 46, which in this example comprises InAlAs orGaInAlAs. Any material which performs the stop etch function to bedescribed can be employed. The layer, 46, is typically 0.1 to 0.5microns thick. Formed on the stop etch layer, 46, is a combinedcladding/SCL layer, 35. In this example, the layer, 35, comprises ann-type layer of InP with a thickness in the range 0.02 to 0.3 microns,and an n-type layer comprising InGaAsP with a thickness in the range0.02 to 0.05 microns. The former layer is a standard cladding layer, andthe latter is a standard SCL layer.

[0021] Formed on the cladding/SCL layer, 35, is an active layer, 36,which in this example is a multi-quantum well layer comprising InGaAsPlayers of different composition so as to form layers of active quantumwell material separated by barrier layers according to principles wellknown in the art. The layer, 36, is typically undoped (intrinsic). Thetypical thickness of the layer, 36, is 0.09 to 0.2 microns.

[0022] A second cladding/SCL layer, 37, which is similar to the layer,35, but with p-type conductivity, is formed on the active layer, 36. Inthis example, the layer, 37, includes a cladding layer comprising InPwith a thickness within the range 0.2 to 0.7 microns, and an SCL layercomprising InGaAsP with a thickness within the range 0.02 to 0.10microns.

[0023] A mask, 60, is then formed on the surface of the layer, 37, byfirst depositing a suitable material, such as silicon dioxide, and thenpatterning the material by standard photolithographic techniques.

[0024] The portions of the semiconductor layers, 35-37, which areexposed by the mask, 60, are then etched such that the final etchingstops on layer 46 as illustrated in FIG. 4. An etchant, such asHCl:H₃PO₄ is used so that layer 46 is not substantially etched.

[0025] Next, as illustrated in FIG. 5, a new cladding/SCL layer, 35′,active layer 36′, and cladding/SCL layer, 37′ are sequentially formed inthe etched out areas. These layers, 35′-37′ are essentially the same asthe corresponding layers, 35-37, except that the composition and/orthicknesses are optimized for a modulator component (32 of FIG. 2) Ifdesired, a new mask (not shown) can be formed to expose the area whichwill become the SOA component (33 of FIG. 2), the exposed area etched,and then layers 35-37 regrown in the etched out areas to optimize theSOA device in a manner similar to the optimization of the modulatorcomponent. For the sake of exposition, it will be assumed that layers35-37 are already optimized for laser and SOA components in the requiredareas.

[0026] Next, as illustrated in FIG. 6, a second mask, 61, is formed overthe surface of the structure of FIG. 5. Again, the mask can be formed bydepositing a layer of material such as silicon dioxide, and thenpatterning the layer by standard photolithographic techniques. In thisstep, the mask is patterned to expose the areas of the structure whichwill comprise the interconnection portions, 62 and 63, between thelaser, modulator, and SOA components, as well as a portion, 64, at theedge of the device for coupling to some external component (not shown).

[0027] As illustrated in FIG. 7, the portions of the structure exposedby the mask, are etched down to the stop etch layer, 46, i.e., layers35,35′, 36,36′, and 37,37′ are selectively etched. Again, the etchantemployed could be HCl:H₃PO₄ or any other suitable wet or dry etchantwhich etches the layers without affecting the mask or the stop etchlayer. Subsequent to the etching step, as illustrated in FIG. 8, layers43-45 are sequentially grown to form the optical interconnectionportions, 62, 63, and 64 in the etched out areas. The mask, 61, can beused to provide selectivity in the growth process. In this example, thelayers, 44 and 45, were SCL layers comprising InP, typically with athickness in the range 0.005 to 0.6 microns. Layer, 43, sandwichedbetween the SCL layers was a passive waveguide layer comprising InGaAsP.The layers, 43-45 can be grown under conditions which optimizeinterconnection independently of the conditions for optimizing thedevice components. For example, the passive waveguide must be aligned tothe two dissimilar device active regions such that low loss modetransfer occurs.

[0028] The mask. 61, can then be removed, and the structure completed bydepositing the necessary electrodes to the top and bottom surfaces ofthe structure and cleaving the structure according to standardtechniques to produce the device depicted in FIG. 2.

What is claimed is:
 1. An optoelectronic device comprising at least twospatially separate optical components formed on a single semiconductorsubstrate, each optical component including an active region, and apassive waveguide formed over the substrate and optically butt couplingthe two components.
 2. The device according to claim 1 wherein theactive regions comprise multi-quantum well layers.
 3. The deviceaccording to claim 1 comprising at least three optical components ofdifferent types.
 4. The device according to claim 3 wherein the devicecomprises a laser, a modulator, and an optical amplifier.
 5. The deviceaccording to claim 1 wherein each component includes a cladding layer oneither side of the active region, and further comprising a claddinglayer on either side of the passive waveguide.
 6. The device accordingto claim 2 wherein the muli-quantum layers comprise InGaAsP.
 7. Thedevice according to claim 5 wherein each component further comprises aseparate confinement layer on either side of the active region.
 8. Thedevice according to claim 1 further comprising a stop-etch layer formedover the substrate.
 9. The device according to claim 8 wherein thestop-etch layer comprises InAlAs or GaInAlAs.
 10. The device accordingto claim 1 wherein the passive waveguide comprises an identicalcomposition between the components.
 11. An optoelectronic devicecomprising: at least three spatially separate optical componentsincluding a laser, modulator, and optical amplifier formed on a singlesubstrate, each optical component comprising a multi-quantum well layercomprising InGaAsP sandwiched between cladding layers and separateconfinement layers; a passive waveguide formed over the substrate so asto form butt joints with the multi-quantum well layers and opticallyconnect the components, the waveguide having an identical compositionbetween the components comprising InGaAsP; and a stop-etch layercomprising InAlAs or GaInAlAs formed over the substrate.
 12. A method offorming an optoelectronic device comprising the steps of: forming aplurality of epitaxial semiconductor layers on essentially the entiresurface of a semiconductor substrate, the layers including at least onelayer of an active material; selectively etching the layers to formspatially separate structures including the active material; and formingat least one passive waveguide layer in the etched areas so as toprovide optical butt coupling between the active material of theseparate structures.
 13. The method according to claim 12 furthercomprising, prior to forming the active material, forming an etch-stoplayer over the substrate, and selectively etching the epitaxial layersdown to the etch-stop layer.
 14. The method according to claim 13wherein the etch-stop layer comprises InAlAs or GaInAlAs.
 15. The methodaccording to claim 12 wherein a plurality of layers including thepassive waveguide layer are sequentially formed in the etched areas. 16.The method according to claim 12 wherein the active material comprisesInGaAsP, and the passive waveguide comprises InGaAsP.
 17. The methodaccording to claim 12 wherein, prior to forming the passive waveguide, aseparate plurality of epitaxial layers is formed for each type ofoptical component on the substrate.
 18. The method according to claim 15wherein the plurality of layers formed in the etched areas includescladding layers.
 19. The method according to claim 12 wherein thespatially separate structures are formed into at least a laser,modulator, and optical amplifier.
 20. A method of forming anoptoelectronic device including at least a laser, modulator, and opticalamplifier on a single substrate comprising the steps of: forming anetch-stop layer comprising InAlAs or GaInAlAs on a surface of thesubstrate; separately forming a plurality of epitaxial layers over thesubstrate for each of the laser, modulator, and amplifier, the pluralityof layers including multi-quantum active layers comprising InGaAsP;selectively etching the plurality of epitaxial layers down to theetch-stop layer to form spatially separate structures; sequentiallyforming a first cladding layer comprising InP, a passive waveguide layercomprising InGaAsP, and a second cladding layer comprising InP in theetched areas so as to form butt joints between the active layers andpassive waveguide layer; and forming the spatially separate structuresinto the laser, modulator, and optical amplifier.